Generally, cold rolling refers to compressing and squeezing metal strips between work rolls at room temperature, and hot rolling refers to compressing and squeezing metal strips while applying heat.
The advantages of cold rolling are good dimensional accuracy and surface finish. However, cold rolling necessarily requires an annealing process, so the entire rolling process time is lengthened and the productivity is lowered.
In hot rolling, a metal strip is heated in a heating furnace, and is then fed to a rolling device. It is extremely important to heat the metal strip to a specified temperature before rolling. When the heating temperature is much lower than a specified one, many difficulties may occur such that it is hard to carry out rolling, excessive loads are imposed on the rolling device, and desirable properties cannot be obtained for the rolled strip.
The metal strip is cooled by heat radiation while the metal strip is being transferred to the rolling device after being discharged from the heating furnace. The metal strip is further cooled when it contacts work rolls just before rolling. Therefore, the metal strip is heated to a specified temperature which is determined by taking into account such decreases in temperature. However, if an excessively high temperature is set as the heating temperature, there is an increase in oxidization of the metal strip and an increase in energy costs is inevitable.
Under these circumstances, it is advisable to heat metal strips as close as possible to work rolls. For such a purpose, it is conceivable to employ a high-frequency induction heating method and an electric heating method.
However, a high-frequency induction heating device is complicated, expensive and high power-consumptive.
An example of an electric heating type rolling device is disclosed in Japanese patent publication No.1998-180317, which will be described with reference to FIG. 1.
As shown in the drawing, slip rings 12 and 13, which are electrically connected to a power supply 14, are respectively provided at each end of upper and lower work rolls 10. Constant current is supplied to the work rolls 10 and a metal strip S from the power supply 14, and the metal strip S is heated to a predetermined temperature due to its own electric resistance.
However, the prior art electric heating type rolling device has a problem of consuming excessive power. When a steel strip having a width of 100 mm and a thickness of 2 mm, i.e. a cross-sectional area of 2 cm2, is rolled to have a thickness of 0.25˜0.3 mm by electric-heating, a current density of about 104 A/cm2 is required. In case of applying constant current, the current strength reaches to 20000 A, multiplying the current density by the cross-sectional area of the strip to be rolled.
Further to such an excessive power-consumption, the steel strip is heated to a temperature ranging from 400° C. to 500° C., which causes oxidization and discoloration on the strip surface.
Also, because the work rolls are included in the electric circuit, the life of work rolls may be shortened due to electric corrosion, and a cooling device for preventing the work rolls from being damaged by heat transfer from the steel strip is additionally required.